This application claims the benefit of Korean Patent Application No. 2001-50167, which was filed in Korea on Aug. 21, 2001, and which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to X-ray detectors. More particularly, it relates to Thin Film Transistor (TFT) array substrates for use in X-ray detectors.
2. Description of Related Art
A widely used aid to medical diagnosis is the X-ray film. As such films produce photographic images, time consuming film-processing procedures are required to obtain the results. However, digital X-ray sensing devices (referred to hereinafter as X-ray detectors) that employ thin film transistors have been developed. Such X-ray sensing devices have the significant advantage of providing real time imaging that speeds diagnosis.
FIG. 1 is a schematic cross-sectional view illustrating the structure and operation of an X-ray detector according to a conventional art. Included are a substrate 1, a thin film transistor 3, a storage capacitor 2, a pixel electrode 4, a photoconductive film 5, a protection film 6, a conductive electrode 7 and a high voltage D.C. (direct current) power supply 8.
The photoconductive film 5 produces electron-hole pairs 9 in proportion to the strength of external signals (such as incident electromagnetic waves). That is, the photoconductive film 5 acts as a converter that converts external signals, particularly X-rays, into electric signals. When an external voltage Va is applied across the conductive electrode 7, the electron-hole pairs 9 separate such that X-ray induced electrical charges accumulate on the pixel electrode 4. Thus, either the electrons or the holes are gathered by the pixel electrode 4 as electric charges. The electric charge species that is gathered depends on the polarity of the high voltage D.C. power supply 8.
As shown in FIG. 1, the pixel electrode 4 is located beneath the photoconductive film 5. The gathered electric charges accumulate in the storage capacitor 2, which connects with a grounding line. Charges in the storage capacitor 2 are selectively transferred through a thin film transistor (TFT) 3 to an external image display device that produces an X-ray image.
FIG. 2 is a schematic plan view illustrating one pixel of an X-ray detector array substrate according to the conventional art, and FIG. 3 is a cross-sectional view taken along line IIIxe2x80x94III of FIG. 2.
As shown in FIGS. 2 and 3, a gate line 21 is arranged in a transverse direction on a substrate 10. A gate electrode 22 that extends from the gate line 21 is also on the substrate 10. A gate insulation layer 30 is formed over the substrate 10, over the gate line 21, and over the gate electrode 22. An active layer 41, comprised of amorphous silicon, is formed on the gate insulation layer 30 and over the gate electrode 22. Ohmic contact layers 42a and 42b, which are comprised of doped amorphous silicon, are formed on the active layer 41.
A data line 51 is on the gate insulation layer 30. That data line is arranged perpendicular to the gate line 21. A source electrode 52 extends from the data line 51 over the first ohmic contact layer 42a. A drain electrode 53 is on the second ohmic contact layer 42b. The drain electrode 53 is spaced apart from the source electrode 52 such that the source and drain electrodes 52 and 53 face each other across the active layer 41. Therefore, a thin film transistor (TFT) T1 comprised of the gate electrode 22, the active layer 41, the ohmic contact layers 42a and 42b, and the source and drain electrodes 52 and 53 is formed as a switching element near the crossing of the gate and data lines 21 and 51.
Still referring to FIGS. 2 and 3, a common line 55, which is comprised of the same material as the data line 51, is arranged perpendicularly to the gate line 21 so as to cross the pixel region defined by the gate and data lines 21 and 51. The common line 55 grounds the neighboring pixels. A first capacitor electrode 61, made of a transparent conductive material, is formed in the pixel region and on the common line 55. A second capacitor electrode 75, which is also made of a transparent conductive material, is formed over the first capacitor electrode 61. The second capacitor electrode 75 generally corresponds in size and position to the first capacitor electrode 61. A dielectric layer 71 is interposed between the first capacitor electrode 61 and the second capacitor electrode 75, thus forming a storage capacitor C1. A passivation layer 80 is formed on the second capacitor electrode 75 and over the TFT T1 such that the passivation layer 80 protects the storage capacitor C1 and the TFT T1. The passivation layer 80 includes a first contact hole 81, which exposes a portion of the second capacitor electrode 75, and a second contact hole 82, which exposes a portion of the drain electrode 53. As shown in FIG. 3, the second contact hole 82 penetrates both the passivation layer 80 and the dielectric layer 71.
A pixel electrode 91, which is made of a transparent conductive material, is formed on the passivation layer 80 in the pixel region. The pixel electrode 91 extends over the TFT T1. The pixel electrode contacts the second capacitor electrode 75 through the first contact hole 81 and contacts the drain electrode 53 through the second contact hole 82. Although not shown in FIG. 3 (but see FIG. 6I for similar structures), a photoconductive film that generates electric charges is on the pixel electrode 91. The pixel electrode 91 gathers electric charges generated by the photoconductive film and applies them to the storage capacitor C1. Namely, the pixel electrode 91 acts as a current-collecting electrode. As previously mentioned, since the pixel electrode 91 electrically contacts the drain electrode 53 through the second contact hole 82, the holes stored in the storage capacitor C1 combine with the electrons that flow from the TFT T1.
When the X-ray image sensing device produces image signals, the electric charges stored in the storage capacitor C1 flow to TFT T1 by way of the pixel electrode 91, which contacts both the first capacitor electrode 75 an one through the first contact hole 81 and the other through the second contact hole 82, the pixel electrode 91 contact resistance is relatively large. Therefore, weak signals, which produce a small quantity of electric charges, are difficult to detect because it is difficult to distinguish between the actual signals and noise. Thus, the sensitivity of the X-ray detector is less than optimal.
Therefore, an X-ray sensing device array substrate having decreased pixel electrode contact resistance would be beneficial. Also beneficial would be an X-ray detector array having improved detection ability.
Accordingly, the present invention is directed to an array substrate for an X-ray detector that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An advantage of the present invention is a method and an X-ray sensing device array substrate having decreased drain electrode contact resistance.
Another advantage of the present invention is a method and X-ray detector array having improved detection ability.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from that description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In order to accomplish at least one of the above advantages, the principles of the present invention provide for an innovate X-ray detector array substrate. That array substrate includes a substrate; a gate line on the substrate; a gate insulation layer over the substrate and over the gate line; and a data line on the gate insulation layer that crosses the gate line so as to define a pixel region. A thin film transistor is located near the crossing of the gate and data lines. The thin film transistor includes a gate electrode, an active layer, a source electrode, a drain electrode and the gate insulation layer, which covers the gate electrode. A ground line crosses the pixel region. A first capacitor electrode is in the pixel region and on the gate insulation layer. The first capacitor electrode covers and contacts the ground line. A dielectric layer covers the first capacitor electrode and the thin film transistor. A first contact hole through the dielectric layer exposes a portion of the drain electrode. A second capacitor electrode is in the pixel region and on the dielectric layer. The second capacitor electrode corresponds to the first capacitor electrode and extends over the drain electrode so as to contact the drain electrode through the first contact hole. A first passivation layer is on the second capacitor electrode and over the thin film transistor. A second contact hole that exposes a portion of the second capacitor electrode passes through the first passivation layer. A pixel electrode is in the pixel region and on the first passivation layer. The pixel electrode contacts the second capacitor electrode through the second contact hole and extends over both the thin film transistor and the gate line. A photoconductive film covers the pixel electrode. A conductive electrode is on the photoconductive film. The thin film transistor beneficially includes first and ohmic contact layers on the active layer. The array substrate further includes a second passivation layer between the photoconductive film and the conductive electrode. The gate line and the gate electrode are beneficially comprised of aluminum (Al) or an aluminum alloy. The gate insulation layer and the dielectric layer include are beneficially comprised of an inorganic substance. The first and second capacitor electrodes and the pixel electrode beneficially include a transparent conductive material, such as indium tin oxide or indium zinc oxide. The first passivation layer beneficially includes benzocyclobutene (BCB).
To achieve the above advantages, the principles of the present invention further provide for a method of fabricating an array substrate for use in an X-ray sensing device. The method includes providing a substrate, forming a gate line on the substrate; forming a gate insulation layer on the substrate and over the gate line, and forming a data line on the gate insulation layer that crosses the gate line, thereby defining a pixel region. The method further includes forming a thin film transistor near the crossing of the gate and data lines, with the thin film transistor including a gate electrode, an active layer, a source electrode, a drain electrode and a gate insulation layer, wherein the gate electrode is covered by the gate insulation layer. The method further includes forming a ground line that crosses the pixel region, forming a first capacitor electrode on the gate insulation layer in the pixel region such that the first capacitor electrode covers and contacts the ground line; and forming a dielectric layer on the first capacitor electrode, with the dielectric layer covering the first capacitor electrode and the thin film transistor, and having a first contact hole that exposes a portion of the drain electrode of the thin film transistor. Then, forming a second capacitor electrode in the pixel region and on the dielectric layer, with the second capacitor electrode corresponding to the first capacitor electrode and extending over the drain electrode so as to contact the drain electrode through the first contact hole. Then, forming a first passivation layer on the second capacitor electrode and over the thin film transistor, with the first passivation layer having a second contact hole that exposes a portion of the second capacitor electrode, and forming a pixel electrode on the first passivation layer in the pixel region, with the pixel electrode contacting the second capacitor electrode through the second contact hole and extending over the thin film transistor and over the gate line. Then, forming a photoconductive film on the pixel electrode such that the photoconductive film covers the pixel electrode. Then, forming a conductive electrode on the photoconductive film. The method further includes locating a second passivation layer between the photoconductive film and the conductive electrode.
In order to accomplish the above advantages, in another aspect, the principles of the present invention provide for a method of fabricating an array substrate for use in an X-ray sensing device. That method includes forming a gate line and a gate electrode on a substrate; forming a gate insulation layer on the substrate so as to cover the gate line and the gate electrode; forming an active layer on the gate insulation layer and over the gate electrode; forming first and second ohmic contact layers on the active layer; forming a data line, a source electrode, a drain electrode and a ground line, thereby defining a thin film transistor, wherein the data line is on the gate insulation layer and perpendicularly crosses the gate to define a pixel region, wherein the source electrode extends from the data line over the first ohmic contact layer, wherein the drain electrode is spaced apart from the source electrode and is on the second ohmic contact layer, and wherein the ground line crosses the pixel region; forming a first capacitor electrode on the gate insulation layer in the pixel region so as to cover and contact the ground line; forming a dielectric layer on the first capacitor electrode and on the thin film transistor, wherein the dielectric layer has a first contact hole that exposes a portion of the drain electrode, forming a second capacitor electrode on the dielectric layer in the pixel region, wherein the second capacitor electrode overlaps and contacts a portion of the drain electrode through the first contact hole; forming a first passivation layer on the second capacitor electrode and on the dielectric layer over the thin film transistor, wherein the first passivation layer has a second contact hole that exposes a portion of the second capacitor electrode; forming a pixel electrode on the first passivation layer in the pixel region, wherein the pixel electrode contacts the second capacitor electrode through the second contact hole; forming a photoconductive film on the pixel electrode; and forming a conductive electrode over the photoconductive film. The method further includes forming a second passivation layer between the photoconductive film and the conductive electrode. The first passivation layer is beneficially comprised of an organic substance, such as benzocyclobutene (BCB). The pixel electrode beneficially extends over the thin film transistor and over the gate line. The pixel electrode and the first and second capacitor electrode are beneficially comprised of a transparent conductive material such as indium tin oxide and/or indium zinc oxide.
To accomplish at least one of the above advantages, in another aspect, the principles of the present invention provide for an X-ray detector array substrate. That array substrate includes a gate line and a gate electrode on a substrate. A gate insulation layer covers the substrate, the gate line, and the gate electrode. An active layer is on the gate insulation layer and over the gate electrode. First and second ohmic contact layers are formed on the active layer. A data line is on the gate insulation layer and crosses the gate line. The data line and the gate define a pixel region. Source and drain electrodes are on the first and second ohmic contact layer, respectively, wherein the source electrode extends from the data line and contacts the first ohmic contact layer, and wherein the drain electrode is spaced apart from the source electrode and is on the second ohmic contact layer, thereby defining a thin film transistor. A ground line crosses the pixel region. A first capacitor electrode is on the gate insulation layer. The first capacitor electrode both covers and contacts the ground line. A dielectric layer is over both the first capacitor electrode and the thin film transistor, wherein the dielectric layer has a first contact hole that exposes a portion of the drain electrode. A second capacitor electrode is on the dielectric layer, wherein the second capacitor electrode overlaps and contacts a portion of the drain electrode through the first contact hole. A first passivation layer is on the second capacitor electrode and on the dielectric layer over the thin film transistor, wherein the first passivation layer has a second contact hole that exposes a portion of the second capacitor electrode. A pixel electrode is on the first passivation layer so as to contact the second capacitor electrode through the second contact hole. A photoconductive film is on the pixel electrode, and a conductive electrode is over the photoconductive film.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.